This dialog asks do you want to permanently delete files that have been in Quarantine for more than 90 days. Files that have been put into Quarantine are not deleted so that they can be restored if necessary. After 90 days it is very likely that they are dispensable so they can be purged to save disk space.
Old quarantined items (Spybot - Search & Destroy 2)
Spybot - Search & Destroy 2
Old quarantined items